Are you using swaylock with a different compositor than Sway? In b9851a9 we moved that option to the Sway module (see also the comment in pam.nix), but for users that use a different compositor this isn't ideal (need to set the option manually - might be a good idea to at least mention this in longDescription of swaylock).

I use Sway as compositor, but I enable it via home-manager, maybe that's the reason, why this option was not set

Yes, that's the case then. IMO the home-manager module should recommend to also enable Sway system-wide (programs.sway.enable) so that all relevant options are set:

    security.pam.services.swaylock = {};
    hardware.opengl.enable = mkDefault true;
    fonts.enableDefaultFonts = mkDefault true;
    programs.dconf.enable = mkDefault true;
    # To make a Sway session available if a display manager like SDDM is enabled:
    services.xserver.displayManager.sessionPackages = [ swayPackage ];

(A default system configuration most likely lacks the first and last option.)

And this shouldn't cause any conflicts (though I'm not familiar with the home-manager module).

I've added a short note to swaylock in 023e11a but unfortunately that will probably not help that much, especially for home-manager.

@SeTSeR I'll close this PR as I don't think there's anything we could/should change in Nixpkgs, but feel free to re-open if you disagree ;) Might be a good idea to open a home-manager PR/issue to document this (maybe a hint in the enable option of the home-manager module that the NixOS module for Sway should also be enabled?).

Forwarded issue to home-manager: nix-community/home-manager#1288